Apparatus and Method for Driving Headphones Differentially in Mobile Applications

ABSTRACT

An apparatus and method is disclosed for achieving improved sound quality from mobile ‘hifi’ playback devices by driving compatible headphones in ‘balanced’ or ‘differential’ mode via standard size headphone connectors on the device, while retaining full compliance with legacy jack connections and conventional headphones. When a headphone is connected, a smartphone may determine whether the headphone is one capable of accepting balanced audio signals, or one that uses a conventional 3-pole jack or a 4-pole CTIA or OMTP jack. For a headphone that accepts balanced audio signals, the four poles of a 4-pole jack are used to drive left and right audio channels, and inverted left and right audio channels. For conventional 3-pole or 4-pole jacks, switches in the smartphone adapt the audio output signals to the configuration expected by the headphone.

This application claims priority from Provisional Application Nos.62/217,585, filed Sep. 11, 2015, and 62/309,924, filed Mar. 17, 2016,which are incorporated by reference herein in their entirety.

FIELD OF THE INVENTION

The present invention relates generally to mobile devices, and moreparticularly to driving headphones through an analog output port in sucha mobile device.

BACKGROUND OF THE INVENTION

It is now common for people to listen to music using mobile devices suchas smartphones or other mobile media players (collectively“smartphones”). Users have rapidly become accustomed to having musicplayed back through such devices, and often tend to have theirsmartphones with them at all times. At least many, if not most, of theseusers want to be able to listen to music having high sound quality.Smartphone makers have recognized this, and many have introduced ‘hifi’smartphones, and even based much of their marketing strategy on theirability to deliver better quality sound reproduction than was availablepreviously.

While smartphones have small loudspeakers, often used for makingtelephone calls and “speakerphone” calls, and may alternatively beconnected to some external loudspeaker, many, if not most, users listento music from their smartphones on headphones. A typical headphone to beused with a smartphone is an analog output device that includes a plugthat fits into an analog port on the smartphone, and one or moreearpieces that contain transducers that are activated by signals fromthe smartphone and produce sound to the user. The term “headphone”includes both over-ear devices that are often referred to as“headphones,” as well as in-ear devices that are often called “earbuds.”

An output audio signal is sent from the smartphone by an audio subsystemin the smartphone to the earpiece(s) in the headphone and activates thetransducers to produce sound so that a user may listen to the audiooutput. The analog port typically contains two output channels, referredto in the art as “Left” and “Right” (and generally intended to go to theleft and right ear of the user, respectively), so as to be capable ofproviding stereo audio output. If the headphone includes two earpiecesand the source material has been recorded in a stereo format, onechannel will go to each earpiece and the user will hear stereo audiooutput. Other devices, such as some eternal loudspeakers and amplifiers,may also have a plug that fits into the analog port to receive theoutput audio in stereo.

While current smartphones are able to reproduce stereo music with highsound quality, they are typically sold with included headphones thattend to be of mediocre or even poor quality, thus limiting the user'smusical experience. It is thus not uncommon for the purchaser of a newsmartphone to almost immediately discard the included headphones andpurchase a set of after-market headphones of higher quality than thoseincluded in order to take full advantage of the ability of thesmartphone to play music with high sound quality.

However, the performance of even high quality headphones is typicallylimited by the performance of the connection from the audio output porton the smartphone to the headphone transducers, and how the audiosignals are provided to the headphones.

The industry-standard analog port for outputting audio from thesmartphone typically has a receptacle known as a “socket” that acceptsan inserted element, most often a 3.5 millimeter (mm) “jack” or plug. (Aless common alternative is a port that accepts a 2.5 mm jack.) Oneversion of the industry standard jack is shown in FIG. 1a . Jack 102 hasthree segments or poles, which are electrically isolated from each otherand connect to separate wires or components of the headphone or otherdevice being plugged into the analog port. The three poles are commonlyknown in the art as the Tip, the Ring, and the Sleeve. The analog portsocket similarly has three segments (not shown) which match up andconnect to the three poles of jack 102 when fully inserted into thesocket, and which are also electrically isolated from each other andconnect to separate components of the smartphone.

FIG. 2 shows the typical way that a stereo signal is output from theanalog port of a smartphone to a 3-pole jack such as 3-pole jack 102 ofFIG. 1. In particular, the jack and socket configuration used instandard headphone connections separates the Left and Right signal pathsonto separate wires. The Left channel audio signal, labeled LP, is fedthrough the Tip portion of jack 102, while the Right channel audiosignal, labeled RP is fed through the Ring of jack 102.

However, since jack 102 only has a 3-pole connection, and two of thepoles are used for the Left and Right outputs, this configuration thusrequires that the ground return signal paths for both channels, labeledLGnd and RGnd, are shared through the Sleeve of the jack, and thenthrough the socket connection to the smartphone and across the internaldevice circuit board.

This common grounding of the channels results in crosstalk and signalloss through the parasitic resistance of this shared signal groundreturn path. While there are some specific circuit implementations thathave been tried to mitigate these issues, for example where feedback tothe amplifiers is taken from as close to the jack common ground point aspossible, ultimately it is not possible to avoid the common grounding ofthe left and right signal return paths in a 3-pole connector.

Returning to FIG. 1b , another version of the industry standard jack isshown, in which a jack 104 now contains 4 poles. Two of the poles aredesignated in the same way as those of 3-pole jack 102 of FIG. 1a ,i.e., the Tip and the Sleeve. The third pole, called the Ring in a3-pole jack, is now called Ring1 to distinguish it from the fourth pole,which is located between Ring1 and the Sleeve and known in the art asRing2. The analog port socket (not shown) similarly contains a fourthsegment that corresponds to Ring2 and connects with Ring2 when jack 102is fully inserted into the socket.

In most mobile communication devices such as smartphones the extra poleon the standard 3.5 mm jack, and the corresponding portion of thesocket, is used to support connection of a microphone in the headset forvoice communication. A headphone including a microphone is sometimescalled a “headset”; as used herein, “headphone” includes headsets. As iswell known in the art, many headsets include a device, sometimes calleda “dongle,” that includes one or more buttons that the user may press toselect certain functions from the audio subsystem in the smartphone,such as to answer a telephone call, pause or play audio, or increase ordecrease the volume of the transducers in the headset. This device istypically located on the cord from the earpieces of the headphone to thejack.

It will be apparent that, although the jack and socket arrangement isnow a 4-pole jack, there is still only one pole available for the groundreturn signals of the Left and Right audio channels, and now for themicrophone as well.

There are two industry-standard configurations of a 4-pole jackpresently in use with smartphones. FIG. 3 is a diagram of a 4-pole jack302 of one configuration. This configuration is known as the CTIAstandard, for the organization that adopted it (originally known as theCellular Telephone Industries Association), or alternatively as theAmerican Headset Jack (AHJ) standard.

In a CTIA configuration, as with the 3-pole configuration of FIG. 2, theleft and right audio channel signals LP and RP are passed to theheadphone through the Tip and Ring1 of jack 302, respectively. Inaddition, the microphone output, which is not present in the 3-poleconfiguration, is passed through the Sleeve portion of jack 302. Theground connector for both the Left and Right channel signals now alsoincludes the microphone signal, and is connected to Ring2.

The second common configuration is the OMTP standard (from the OpenMobile Terminal Platform). FIG. 4 is a diagram of this configuration ofa 4-pole jack 402. As with the CTIA configuration and the 3-poleconfiguration, the left and right audio channel signals LP and RP arepassed to the headphone through the Tip and Ring1 of jack 402,respectively.

As compared to the CTIA configuration, however, the OMTP configurationreverses the connections of the microphone and ground, so that theground signal return paths, again for both audio channels and themicrophone signal, are through the Sleeve, while the microphone signalis passed to the headphone through Ring2.

However, as with the 3-pole jack 202 of FIG. 2, where a 4-pole jack isused, and regardless of whether the 4-pole jack uses the CTIA or OMTPstandard, in each case the level of audio quality is constrained by theuse of a common ground for the left and right channel signal returnpaths, as well as the microphone signal where a microphone is present.Thus, the crosstalk and signal loss problems of the 3-pole jack 102above are still present, and in fact are exacerbated by the fact thatthe common ground signal return path now includes the microphone signalas well as the Left and Right channel signals.

All known smartphone or other mobile audio devices use audio outputsockets configured to accept industry standard 3-pole jacks or CTIA orOMTP 4-pole jacks. Thus, all such known devices provide common groundsignal return paths, and thus suffer from these described audio signalquality issues. It would be advantageous to be able connect high qualityheadphones to mobile devices in such a way as to avoid the need for acommon ground return path for left and right channel audio signals.

SUMMARY OF THE INVENTION

An apparatus and method is described whereby improved sound quality isachieved from mobile ‘hifi’ playback devices by driving compatibleheadphones in ‘balanced’ or ‘differential’ mode via standard sizeheadphone connectors on the device, while retaining full compliance withlegacy jack connections and conventional headphones. An apparatus andmethod for determining the type of headphone or other reproductiondevice connected to the playback device and configuring the playbackdevice to produce an audio output appropriate for the connected deviceis also described.

One embodiment discloses a mobile audio device for producing a balancedstereo signal, comprising: an analog audio output socket having 4 polesand configured to receive a 4-pole jack corresponding to either the CTIAor OMTP standard; a first amplifier for providing a left channel audiosignal to a first pole of the output socket corresponding to a Tip ofthe jack; a second amplifier for providing a right channel audio signalto a second pole of the output socket corresponding to a Ring1 of thejack; a third amplifier for providing an inverted left channel audiosignal of opposite phase to the left channel audio signal to a thirdpole of the output socket; and a fourth amplifier for providing aninverted right channel audio signal of opposite phase to the rightchannel audio signal to a fourth pole of the output socket.

Another embodiment discloses a method of producing a balanced stereosignal from a mobile audio device having an analog audio output socketwith 4 poles and configured to receive a 4-pole jack corresponding toeither the CTIA or OMTP standard, comprising: providing a left channelaudio signal to a first pole of the output socket corresponding to a Tipof the jack; providing a right channel audio signal to a second pole ofthe output socket corresponding to a Ring1 of the jack; providing aninverted left channel audio signal of opposite phase to the left channelaudio signal to a third pole of the output socket; and providing aninverted right channel audio signal of opposite phase to the rightchannel audio signal to a fourth pole of the output socket.

A further embodiment discloses a mobile audio device for producing abalanced stereo signal, comprising: an analog audio output socket having4 poles and configured to receive a 4-pole jack corresponding to eitherthe CTIA or OMTP standard; a first amplifier for providing a leftchannel audio signal; a second amplifier for providing a right channelaudio signal; a third amplifier for providing an inverted left channelaudio signal of opposite phase to the left channel audio signal; afourth amplifier for providing an inverted right channel audio signal ofopposite phase to the right channel audio signal; and a controllerconfigured to: measure the impedances between a first pole of the socketand a second pole of the socket, the first pole and a third pole of thesocket, a fourth pole of the socket and the second pole, and the fourthpole and the third pole; compare the measured impedances to expectedvalues for devices using 3-pole jacks, devices using 4-pole jackscorresponding to the CTIA or OMTP standards, and devices using 4-polejacks that can accept a balanced stereo signal; determine which of thedevices is indicated by the measured impedances, and, if the device isone that can accept a balanced stereo signal, and cause the left channelaudio signal to be provided to a first pole of the output socketcorresponding to a Tip of the jack, the right channel audio signal to beprovided to a second pole of the output socket corresponding to a Ring1of the jack, the inverted left channel audio signal to be provided to athird pole of the output socket, and the inverted right channel audiosignal to be provided to a fourth pole of the output socket.

Still another embodiment discloses a method of producing a balancedstereo signal from a mobile audio device having an analog audio outputsocket with 4 poles and configured to receive a 3-pole jack or a 4-polejack corresponding to either the CTIA or OMTP standard, comprising:measuring the impedances between a first pole of the socket and a secondpole of the socket, the first pole and a third pole of the socket, afourth pole of the socket and the second pole, and the fourth pole andthe third pole; comparing the measured impedances to expected values forattached sound reproduction devices using 3-pole jacks, devices using4-pole jacks corresponding to the CTIA or OMTP standards, and devicesusing 4-pole jacks that can accept a balanced stereo signal; determiningwhich attached sound reproduction device is indicated by the measuredimpedances, and, if the attached sound reproduction device is one thatcan accept a balanced stereo signal: providing a left channel audiosignal to a first pole of the socket corresponding to a Tip of the jack;providing a right channel audio signal to a second pole of the socketcorresponding to a Ring1 of the jack; providing an inverted left channelaudio signal of opposite phase to the left channel audio signal to athird pole of the socket; and providing an inverted right channel audiosignal of opposite phase to the right channel audio signal to a fourthpole of the socket.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1a and 1b are illustrations of the configuration of prior artindustry-standard 3-pole and 4-pole jacks for use with a standard analogport on a mobile device such as a smartphone.

FIG. 2 is an illustration of how signals pass through a prior artindustry-standard 3-pole jack for use with an analog port on a mobiledevice.

FIG. 3 is a diagram of how signals pass through a prior artindustry-standard 4-pole jack for use with an analog port on a mobiledevice using the CTIA standard configuration.

FIG. 4 is a diagram of how signals pass through a prior artindustry-standard 4-pole jack for use with an analog port on a mobiledevice using the OMTP standard configuration.

FIG. 5 shows some components of one embodiment of an audio subsystem ofa smartphone that is capable of producing a balanced output to ahigh-quality headphone while using a standard size 4-pole jack.

FIG. 6 shows an embodiment in which the audio subsystem of a smartphoneas shown in FIG. 5 may be used to provide audio to a standard 4-polejack and headphone having a CTIA configuration.

FIG. 7 shows an embodiment in which the audio subsystem of a smartphoneas shown in FIG. 5 may be used to provide audio to a standard 4-polejack and headphone having an OMTP configuration.

FIG. 8 shows an embodiment in which the audio subsystem of a smartphoneas shown in FIG. 5 may be used to provide audio to a standard 3-polejack and headphone without a microphone.

FIG. 9 is a flowchart of a method for operating the audio subsystem of asmartphone that can support balanced headphones according to oneembodiment.

DETAILED DESCRIPTION OF THE INVENTION

Described herein is an apparatus and method whereby improved soundquality may be achieved from mobile ‘hifi’ playback devices by drivingcompatible headphones in a way that the left and right channel signalshave separate ground return paths via standard size headphone connectorson the device rather than having a common ground return path, whileretaining full compliance with legacy jack connections and conventionalheadphones. The type of headphone or other reproduction device connectedto the playback device may be determined by a controller and theplayback device configured to produce an audio output appropriate forthe connected device.

Some high-quality headphones used for applications other than mobiledevices have separate left and right channel signal return paths, ratherthan being connected in common as in mobile devices. Audio equipmentprofessionals commonly refer to the use of separate signal return pathsas a “balanced” or “differential” connection; such high-qualityheadphones that have separate signal return paths are referred to as“balanced headphones” herein.

Balanced headphones improve the sound quality for the user byeliminating the crosstalk and signal loss caused by common signal returnpaths as described above. In addition, the return signals may beactively driven in opposite phase to the left and right channel signals;this is sometimes also known as a “fully balanced” connection, asdistinguished from separate signal return paths only. As used herein, abalanced connection includes a fully balanced connection as well.

Such prior art balanced headphones are intended for use withnon-portable stereo systems, and use what is known as an XLR styleconnector. However, in the first instance the XLR connector isphysically incompatible with smartphones (or other mobile devices).Further, the sockets used with balanced headphones are dedicated to thatpurpose, and do not provide the signals used by, or accept microphonesignals from, CTIA or OMTP headphones.

It may thus be seen that connecting balanced headphones to a mobiledevice capable of providing separate signal return paths would improvethe sound quality for the user. Again, however, the required signals aredifferent from, and at least partly incompatible with, those used byconventional headphones designed for use with mobile devices.

Since a 4-pole jack by definition has 4 separate signal paths available,such a jack can be configured in a way that drives the desirable“balanced” left and right signal pairs into the headphone totallyseparately, providing higher quality sound than the conventional CTIA orOMTP configurations. In order to support this connection, the headphonemust include a matching 4-pole jack. Because there are 4 poles, both 3.5mm and 2.5 mm 4-pole jack and socket configurations can support thisconnection, although not in the conventional CTIA or OMTPconfigurations.

FIG. 5 shows some components of one embodiment of an audio subsystem ofa smartphone that is capable of producing a balanced output to ahigh-quality headphone while using a standard size 4-pole jack 502. Theleft audio channel signal LP is connected to the Tip of jack 502, andthe right audio channel signal RP is connected to Ring1 as in the priorart discussed above. These audio channel signals LP and RP are producedin the same fashion as in the prior art.

In addition, the audio production subsystem in the smartphone also hasinverters 504, which invert the left and right audio channel signals LPand RP, and produce “inverted” left and right channel audio signals LNand RN, i.e., signals which are of opposite phase to left and rightaudio channel signals LP and RP, respectively. The inverted left audiochannel signal LN is connected to Ring2 of jack 502, and the invertedright audio channel signal RN is connected to the Sleeve.

The inverters 504 are connected to Ring2 and the Sleeve by switches 506,which are not visible in FIG. 5 as they are in a closed position. Theirfunction will be explained further below.

In the “fully balanced” configuration of FIG. 5, as described above, theinverted audio channel signals LN and RN also function as separateground return paths of the left channel signal LP through Ring2, and ofthe right channel RP through the Sleeve. It may thus be seen that whilethe configuration of plug 502 of FIG. 5 allows the use of a standard4-pole jack and socket, neither the audio source nor the headphone aredesigned or operating in a CTIA or OMTP configuration, but rather in thebalanced configuration described.

In the illustrated embodiment, switches 508 and 510 are also providedfor use as described below. When a balanced headphone is used and audiooutput is produced, switches 508 and 510 are open as shown so that theinverted audio channel signals LN and RN are not connected to ground butare passed to jack 502 as described above.

Note that while the configuration of FIG. 5 preserves the standardconnection of the left channel audio signal LP to the Tip and the rightchannel audio signal RP to Ring1, it is arbitrary whether the invertedleft channel signal LN is connected to Ring2 and the inverted rightchannel signal RN to the Sleeve or vice versa. As illustrated, FIG. 5shows a “LRLR” configuration since the left and right channel signalsalternate along the jack 502. It will be clear that switching theconnection of the inverted left channel signal LN to the sleeve, and theinverted right channel signal RN to Ring2, will function in the samefashion; such a configuration may be thought of as a “LRRL”configuration since the two right channel signals will be located onadjacent Ring1 and Ring2 portions of the jack.

As with balanced headphones used in non-smartphone applications,providing these audio signals from a smartphone to a balanced headphonewill result in better audio quality for the user than that provided byconventional headphones. As above, this is due to the elimination ofcrosstalk and the signal loss due to parasitic resistance discussedabove.

However, even if smartphones are manufactured with the ability toprovide balanced audio signals, it is expected that some users willchoose to use conventional headphones rather than high quality balancedheadphones. This may be due to the increased cost of balanced headphonesas compared to conventional headphones, or alternatively to the factthat a balanced headphone does not support a microphone.

It is thus apparent that it will also be desirable to make a smartphonewhich is capable of providing balanced audio signals also capable ofoperating with other standard, non-balanced headphones in theconventional ways described above. To accomplish this, the audioproduction subsystem of the smartphone may be designed to allow forproper operation with any standard headphone by configuring theconnections to the socket so as to feed appropriate signals to eachdifferent type of headphone.

FIG. 6 shows an embodiment in which the components of the audiosubsystem of a smartphone as shown in FIG. 5 may be used to provideaudio to a standard 4-pole jack 602 and headphone having the CTIAconfiguration as shown in FIG. 3 above. As in the configuration of FIG.3, in FIG. 6 the left audio channel signal LP is connected to the Tip ofjack 602, and the right audio channel signal RP is connected to Ring1,as in the prior art described above.

In this instance, however, switches 506 are opened so that invertedsignals from inverters 504 are not provided to Ring2 and the Sleeve,respectively, since the inverted signals are not used by a CTIAheadphone. Rather, as in the standard CTIA configuration shown in FIG.3, the microphone signal MIC is provided through the Sleeve, and thereturn ground signals for both the left and right audio channels and themicrophone are passed through Ring2 and to ground through closed switch508. Switch 510 is opened so that the microphone signal is connected tothe sleeve and not to ground.

In this way, the connection of FIG. 6 allows a smartphone having theability to produce a balanced signal to a high quality headphone to alsoprovide signals appropriate for a headphone including the use of amicrophone as set forth in the CTIA standard above.

FIG. 7 shows an embodiment in which the audio subsystem of a smartphoneas shown in FIG. 5 may similarly be used to provide audio to a standard4-pole jack 702 and headphone having the OMTP configuration as shown inFIG. 4 above. As in the OMTP configuration of FIG. 4, in FIG. 7 the leftaudio channel signal LP is connected to the Tip of jack 702, and theright audio channel signal RP is connected to Ring1, again as in theprior art discussed above.

As with the use of a CTIA headphone and jack as in FIG. 6, switches 506are opened so that no inverted signals from inverters 504 are providedto Ring2 and the Sleeve, respectively. Now, as in the standard OMTPconfiguration shown in FIG. 4, the microphone signal MIC is providedthrough Ring2, and the return ground signals for both the left and rightaudio channels and the microphone are passed through the Sleeve and toground through closed switch 510. Switch 508 is opened so that themicrophone signal is connected to Ring2 and not to ground.

In this way, the connection of FIG. 7 allows a smartphone having theability to produce a balanced signal to a high quality headphone to alsoprovide signals appropriate for a headphone including the use of amicrophone as set forth in the OMTP standard above.

As with the CTIA and OMTP configurations of FIGS. 3 and 4 above, it willbe seen that the configurations of FIGS. 6 and 7 are similar, with theconnections to Ring2 and the Sleeve being reversed, and correspondingswitches open rather than closed and vice versa as appropriate.

FIG. 8 shows an embodiment in which the audio subsystem of a smartphoneas shown in FIG. 5 may similarly be used to provide audio to a standard3-pole jack 802 and corresponding headphone with no microphone. As inthe prior art configuration of FIG. 3, in FIG. 8 the left audio channelsignal LP is connected to the Tip of jack 802, and the right audiochannel signal RP is connected to Ring1.

As with the use of the prior art CTI and OMTP headphones above, switches506 are opened so that no inverted signals from inverters 504 areprovided to Ring2 and the Sleeve, respectively. Since there is only a3-pole jack 802, both the left and right channel ground return signalspass through the Sleeve of jack 802, and thus switches 508 and 510 areboth closed to provide the connections for both signals to ground.

In this way, the connection of FIG. 8 allows a smartphone having theability to produce a balanced signal to a high quality headphone to alsoprovide signals appropriate for a 3-pole headphone that does not includea microphone.

It can thus be seen that an appropriately designed audio subsystem in asmartphone may be capable of driving compatible headphones in ‘balanced’or ‘differential’ mode via standard size headphone connectors on thedevice, as well as retaining full compliance with legacy jackconnections and conventional headphones and providing appropriatesignals to such headphones.

It is desirable that the user be able to plug any headphone into theaudio port socket of a smartphone and hear the audio output without anyselection by the user of the type of headphone connected. Thus, theaudio subsystem should preferably be able to automatically detect that ajack has been inserted into the socket, and to determine what type ofheadphone has been connected.

The audio subsystem must first be able to detect that a headphone hasbeen connected, i.e., that a jack has been inserted into the audiooutput socket. This detection is known in the art, and is typically doneby the use of one or more “jack detect pins.” The jack detect pins areextra contacts inside the socket, which act as switches. The pins onlysense that the plug is inserted, and are not intended for the audiosignal.

When a jack is not present, the jack detect pins, or switches, areclosed; when the jack is inserted, the jack flexes some of the jackdetect pins, and they break contact with other of the jack detect pinsso that the switches are open. The system depends on the mechanicaldeformation of the jack detect pins to break the connections between thejack detect pins and open the switches. Thus, for example, a 3.5 mmplastic rod could be inserted into the socket and still open thecontacts, and the smartphone will think that headphones are plugged in.

Once the insertion of a jack has been detected, as discussed above thereare several types of devices that a smartphone will preferably be ableto detect. One way to detect the connected device that is well known inthe art is by measuring the impedance between the various parts of thesocket that connect to the jack. This may be done by applying a signalof a known voltage to the various parts of the socket using detectionpins (not shown) and measuring the current that flows in response to thesignal. One of skill in the art will appreciate that if the signal is ata frequency either below or above the normal range of human hearing, thesignal will be inaudible to the user.

The first type of device is a standard stereo headphone using a 3-polejack as described above. These devices typically have an impedance ofbetween 6 ohms and 600 ohms from the Tip to the Sleeve, and the samefrom Ring1 to the Sleeve. Further, there is no separate Ring2 in a3-pole jack, and an attempt to measure the impedance from the Tip orRing1 to Ring2 will result in measuring the impedance from the Tip orRing1 to the Sleeve and again a value of 6 to 600 ohms. A commonimpedance value for such headphones is 32 ohms.

The second type of device is a headphone with microphone (i.e., aheadset) using a 4-pole jack in either the standard CTIA or OMTPconfiguration. Again the impedance from the Tip to ground, and fromRing1 to ground, will typically be between 6 and 600 ohms, while theimpedance from the microphone contact to ground will typically bebetween 1000 and 5000 ohms. As above, which of the microphone and groundis connected to Ring2 and which to the Sleeve depends upon whether theheadphone uses the CTIA of OMTP configuration.

Thus, in a CTIA configuration, where Ring2 is the ground connection, theimpedance from the Tip (left channel) to Ring2 (ground), and from Ring1(right channel) to Ring2 (ground), will be 6 to 600 ohms. Since themicrophone is connected to the Sleeve but not to ground, the impedancesfrom the Tip to the Sleeve and from Ring1 to the Sleeve will look likeopen switches and thus be of high (effectively infinite) impedance.

In the OMTP configuration, again the microphone and ground connectionsare reversed, so the impedance from the Tip to the Sleeve (now ground),and from Ring1 to the Sleeve, will be 6 to 600 ohms, while the impedancefrom the Tip to Ring2 (now the microphone), and from Ring1 to Ring2,will be high or open.

The third type of device to be detected is a balanced headphone. In the“LRLR” embodiment of the present application shown in FIG. 5, theimpedance between each channel signal and its return signal ground,i.e., Tip to Ring2 and Ring1 to sleeve, will similarly be between 6 and600 ohms. The other impedances, between the Tip and the Sleeve, andbetween Ring1 and Ring2, will be high or open. (It will be seen that ifthe Ring2 and Sleeve connections are reversed so that the connections tothe jack are in the “LRRL” configuration described above, the impedancemeasurements will similarly be reversed.)

It may thus be seen that by measuring these impedances, i.e., Tip toRing2, Tip to Sleeve, Ring1 to Ring2, and Ring1 to Sleeve, these varioustypes of headphones may be distinguished. Table 1 summarizes whichmeasurements indicate which headphones (Hi Z means high impedance oropen circuit):

TABLE 1 3-pole 4-pole headset Balance headphone Measure headphone CTIAOMTP LRRL LRLR Tip to Ring2 6-600 ohms 6-600 ohms Hi Z Hi Z 6-600 ohmsTip to Sleeve 6-600 ohms Hi Z 6-600 ohms 6-600 ohms Hi Z Ring1 to Ring26-600 ohms 6-600 ohms Hi Z 6-600 ohms Hi Z Ring1 to Sleeve 6-600 ohms HiZ 6-600 ohms Hi Z 6-600 ohms

Currently smartphones are able to distinguish between conventional3-pole and 4-pole jacks and their associated devices, and between CTIAand OMTP devices, by incorporating a circuit that measures the variousimpedances between portions of the jack. One example of a commercialproduct having such capability that may be incorporated in a smartphoneis a TS3A227E Autonomous Audio Accessory Detection and ConfigurationSwitch from Texas Instruments Incorporated.

It will be obvious to one of skill in the art how to modify the circuitof the Texas Instruments chip to detect the different impedances thatwill also determine whether balanced headphones have been connected tothe analog port socket, as well as the conventional 3-pole and 4-poleCTIA and OMTP jacks of the prior art. The impedances are measured in thesame way, but balanced headphones, rather than conventional headphones,are detected when the impedance measurements are as above

FIG. 9 is a simplified flowchart of a method of operating the audiosubsystem of a smartphone that can support balanced headphones asdescribed above. Initially the output port socket of the audio subsystemis idle. (Note that the speakers of the smartphone may be in use, forexample, if the user is making a telephone call without headphones, butthe socket is not activated until a jack is inserted.)

At step 902, the audio subsystem determines whether a jack has beeninserted into the output port socket, typically by using jack detectpins as described above.

Once a jack is detected, at step 904 the audio subsystem will measurethe impedances between portions of the jack as described above. Again,this is typically clone by driving the connections with a signal that isbelow or above the nominal frequency limits of human hearing, so thatthe user will not hear the test signal if the user has the headphoneson.

Once the impedances are known, at step 906 the audio subsystem checks tosee if the type of headphone has been determined. This is done bycomparing the measured impedances to the expected values of each type asshown in Table 1 above. The expected values may, for example, be storedin a lookup table.

If no match to the measured impedances is found, the audio subsystemreturns to the jack detect step 902 (or alternatively to the measureimpedances step 904) to try again to identify the type of jack that hasbeen inserted into the socket.

If there is a match and the type of headphone that is connected has beenidentified, then at step 908 the audio subsystem configures the switchesby opening and/or closing switches as needed to arrive at theappropriate configuration shown in FIG. 5, 6, 7 or 8 above, dependingupon whether the headphone is a balanced headphone, CM headphone, OMTPheadphone, or stereo headphone with a 3-pole plug, respectively.

At step 910, the audio subsystem determines whether a microphone isdetected, i.e., whether the headphone is a CTIA or OMTP headphoneincluding a microphone. If a microphone is detected, at step 912 aportion of the audio subsystem (not shown) is enabled that detects anysignals from the device or “dongle” that may be included in theheadphone cord. This is well known in the art, and is typically done bysensing changes to the microphone impedance that are caused by theswitching in of additional load resistors as the buttons are pressed.

At step 914, the audio subsystem determines whether the user hasselected a desired audio output. This may be in the form of a telephonecall, music that is stored on the smartphone (or part of a videosimilarly stored), or an audio stream from the internet. If an audioselection has been made, the audio subsystem delivers the selected audioto the headphone at step 916; if no audio selection has been made, theaudio subsystem waits for a selection.

It will be apparent to one of skill in the art that certain of thedescribed steps of the method of FIG. 9 in practice may actually requiremultiple sub-steps to implement in a smartphone. One of skill in the artwill further appreciate that in various embodiments the described stepsmay be performed in a slightly different order, or in some casessimultaneously.

The disclosed system and method has been explained above with referenceto several embodiments. Other embodiments will be apparent to thoseskilled in the art in light of this disclosure. Certain aspects of thedescribed method and apparatus may readily be implemented usingconfigurations or steps other than those described in the embodimentsabove, or in conjunction with elements other than or in addition tothose described above.

For example, the described invention may also be used with a USB-Cconnector. As is known in the art, the USB-C connector (not shown) uses24 pins in a 2-row×12-pin configuration and supports connection ofdigital interfaces using USB 3.1 or USB 2.0 protocols, as well asincluding power supply connections. The connector is designed to bereversible, so that a pin in either row has a corresponding duplicatethat is diagonally opposed in the other row. Four pins in the center ofthe USB connector are used for audio signals, with the pins designatedas A6 and B6 both carrying the right audio channel and pins A7 and B7both carrying the left audio channel. As with other prior artconnectors, the return path for both channels is a common ground path.

It will be apparent to one of skill in the art in light of the teachingsherein that, since there are four pins carrying audio signals, ratherthan having two pairs of two pins each carry duplicate signals,alternatively two pins may carry the normal left and right audio channelsignals, and the other two pins may carry the left and right channelsignals of opposite phase as described above. In such a case, theconnector will not be completely reversible, but reversing the connectorwill only result in all four audio signals being reversed in phase,which few users will be able to notice.

In other embodiments, the analog output port of a smartphone may also beconnected to a line input of another device, such as the amplifier of anexternal stereo system. In such a case, the load impedances for the leftand right channel signals might typically be from about 10,000 ohms to100,000 ohms, with 22,000 ohms being common. Similarly, a connectedheadphone might be a noise canceling headphone, in which case theconnections will be the same as either the 3-pole or 4-pole CTIA or OMTPheadphones discussed above, but the load impedance on the left and rightchannels is higher due to an amplifier that is integrated into theheadphone, with 10,000 ohms being typical.

Where elements are shown as connected, they may in some embodiments becoupled to each other through another element or component. One of skillin the art will also appreciate how to determine parameters of thecomponents depending on other components in the smartphone or mobiledevice.

These and other variations upon the embodiments are intended to becovered by the present disclosure, which is limited only by the appendedclaims.

What is claimed is:
 1. A mobile audio device for producing a balancedstereo signal, comprising: an analog audio output socket having 4 polesand configured to receive a 4-pole jack corresponding to either a CTIAor OMTP standard; a first amplifier configured to provide a left channelaudio signal to a first pole of the output socket corresponding to a Tipof the jack; a second amplifier configured to provide a right channelaudio signal to a second pole of the output socket corresponding to aRing1 of the jack; a third amplifier configured to provide an invertedleft channel audio signal of opposite phase to the left channel audiosignal to a third pole of the output socket; and a fourth amplifierconfigured to provide an inverted right channel audio signal of oppositephase to the right channel audio signal to a fourth pole of the outputsocket.
 2. The mobile audio device of claim 1, wherein the socket issized to receive a standard 3.5 mm jack.
 3. The mobile audio device ofclaim 1, wherein the socket is sized to receive a standard 2.5 mm jack.4. The mobile audio device of claim 1 wherein the analog output socketis configured to receive a 4-pole jack corresponding to the CTIAstandard, and the third pole of the socket corresponds to a Ring2 of thejack and the fourth pole of the socket corresponds to a Sleeve of thejack.
 5. The mobile audio device of claim 1 wherein the analog outputsocket is configured to receive a 4-pole jack corresponding to the OMTPstandard, and the third pole of the socket corresponds to a Sleeve ofthe jack and the fourth pole of the socket corresponds to a Ring2 of thejack.
 6. The mobile audio device of claim 4, further comprising switchesfor disconnecting the outputs of the third and fourth amplifiers fromthe third and fourth poles of the socket, a switch for connecting thepole of the socket corresponding to Ring2 of the jack to ground, and aswitch for connecting the pole of the socket corresponding to the Sleeveof the jack to a microphone.
 7. The mobile audio device of claim 5,further comprising switches for disconnecting the outputs of the thirdand fourth amplifiers from the third and fourth poles of the socket, aswitch for connecting the pole of the socket corresponding to the Sleeveof the jack to ground, and a switch for connecting the pole of thesocket corresponding to Ring2 of the jack to a microphone.
 8. The mobileaudio device of claim 1, further comprising switches for disconnectingthe outputs of the third and fourth amplifiers from the third and fourthpoles of the socket, and switches for connecting the poles of the socketcorresponding to a Ring2 and a Sleeve of the jack to ground, such thatthe audio device delivers a stereo signal through a 3-pole jack.
 9. Amethod of producing a balanced stereo signal from a mobile audio devicehaving an analog audio output socket with 4 poles and configured toreceive a 4-pole jack corresponding to either a CTIA or OMTP standard,comprising: providing a left channel audio signal to a first pole of theoutput socket corresponding to a Tip of the jack; providing a rightchannel audio signal to a second pole of the output socket correspondingto a Ring1 of the jack; providing an inverted left channel audio signalof opposite phase to the left channel audio signal to a third pole ofthe output socket; and providing an inverted right channel audio signalof opposite phase to the right channel audio signal to a fourth pole ofthe output socket.
 10. The method of claim 9 wherein the analog outputsocket is configured to receive a 4-pole jack corresponding to the CTIAstandard, providing an inverted left channel audio signal furthercomprises providing the inverted left channel audio signal to a thirdpole of the output socket corresponding to a Ring2 of the jack, andproviding an inverted right channel audio signal further comprisesproviding the inverted right channel audio signal to a fourth pole ofthe output socket corresponding to a Sleeve of the jack.
 11. The methodof claim 9 wherein the analog output socket is configured to receive a4-pole jack corresponding to the OMTP standard, providing an invertedleft channel audio signal further comprises providing the inverted leftchannel audio signal to a third pole of the output socket correspondingto a Sleeve of the jack, and providing an inverted right channel audiosignal further comprises providing the inverted right channel audiosignal to a fourth pole of the output socket corresponding to a Ring2 ofthe jack.
 12. The method of claim 10, further comprising disconnectingthe outputs of the third and fourth amplifiers from the third and fourthpoles of the socket, connecting the pole of the socket corresponding toRing2 of the jack to ground, and connecting the pole of the socketcorresponding to the Sleeve of the jack to a microphone.
 13. The methodof claim 11, further comprising disconnecting the outputs of the thirdand fourth amplifiers from the third and fourth poles of the socket,connecting the pole of the socket corresponding to the Sleeve of thejack to ground, and connecting the pole of the socket corresponding toRing2 of the jack to a microphone.
 14. The method of claim 9, furthercomprising disconnecting the outputs of the third and fourth amplifiersfrom the third and fourth poles of the socket, and connecting the polesof the socket corresponding to a Ring2 and a Sleeve of the jack toground, such that the audio device delivers a stereo signal through a3-pole jack.